Laser materials processing as known in the art and used herein refers to performance of materials processes such as cutting, welding, drilling and soldering, using a continuous wave or pulsed laser beam. The average power of such a laser beam may range from as little as approximately one watt to hundreds of watts. It is also known in the art to transmit the laser beam from the laser to the vicinity of the workpiece by means of an optical fiber. The apparatus and method for injecting a power laser beam into an optical fiber for transmission therethrough are disclosed in commonly assigned U.S. Pat. Nos. 4,564,736; 4,676,586; and 4,681,396 respectively entitled "Industrial Hand Held Laser Tool and Laser System", "Apparatus and Method for Performing Laser Material Processing Through a Fiber Optic", and "High Power Laser Energy Delivery System" the disclosures of those patents being incorporated in their entirety herein by reference.
In order to perform materials processing with the laser beam emitted from an output end of the transmitting optical fiber, it is necessary to terminate the fiber output end in an output coupler. A typical prior art output coupler 100 is diagrammatically illustrated in FIG. 1. Coupler 100 comprises a fiber holder 102 to support the output end of an optical fiber 104 through which the power laser beam is being transmitted. The core of fiber 104 has a diameter d.sub.f. The beam is emitted from the fiber output end, with an emitted cone angle .theta..sub.EM, as an emitted beam portion 106. The output coupler further comprises a lens 108 for collimating as a collimated beam portion 110 the power laser beam emitted from the fiber output end. Collimating lens 108 has a focal length f.sub.1 and is positioned the distance f.sub.1 from the end of fiber 104. A second lens 112 is provided for focussing the collimated beam portion as a focussed beam portion 114. Focussing lens 112 has a focal length f.sub.2. The focussed portion is focussed as a spot 116 onto a workpiece 118 on which a desired materials process is to occur. The coupler is assumed to be positioned relative to the workpiece to provide the distance f.sub.2 between lens 112 and spot 116. In materials processing, irrespective of whether the laser beam is directly delivered from the laser to the vicinity of the workpiece by optical hardware (lenses, mirrors, etc.) or through an optical fiber, it is generally desirable to maximize the beam power density at the focussed spot on the workpiece. The power density achieved on the workpiece, such as at spot 116 in FIG. 1, is determined by dividing the amount of power contained in the delivered beam by the area of the focussed spot. Power density maximization is desirable since most laser materials processes, e.g. welding, will not occur below a minimum required power density and such processes will proceed with increased speed in proportion to the amount by which the focussed spot power density exceeds the minimum required power density.
One drawback to transmission of a power laser beam through an optical fiber is the adverse effect of the transmission on beam power and quality. The fiber transmitted laser beam experiences power losses during transmission through the fiber. To the extent total beam power is reduced, the focussed spot power density at the workpiece is correspondingly reduced. The adverse effect of optical fiber beam transmission on beam quality is described in connection with FIG. 2. FIG. 2 diagrammatically illustrates the injection of a collimated laser beam 150, generated by a laser 152, into the same optical fiber 104 illustrated in FIG. 1 for transmission therethrough. The collimated laser beam is focussed by a focussing lens 154, as a focussed portion 156, onto an input end of fiber 104 as a focussed spot having a diameter "S.sub.F ". Focussed portion 156 is characterized by an entry cone angle .theta..sub.ENT. The focal length of lens 154 is f.sub.154. In accordance with the criteria for successful power laser beam injection into an optical fiber as taught in the above incorporated patents, the focussed spot diameter S.sub.F must be less than the diameter d.sub.f of fiber 104. The beam quality of the laser beam as generated by laser 152 is BQ.sub.L and can be expressed as: EQU BQ.sub.L =S.sub.F .times..theta..sub.ENT ( 1)
where the distance between lens 154 and the input end of the fiber is f.sub.154. At the output end of fiber 104, the transmitted beam is emitted with the emitted cone angle of .theta..sub.EM. The emitted beam fully occupies the diameter d.sub.f of the fiber core, so that the beam quality BQ.sub.F of the laser beam emitted at the fiber output end can be expressed as: EQU BQ.sub.F =d.sub.f .times..theta..sub.EM ( 2)
The inventor has observed that .theta..sub.EM is usually greater than .theta..sub.ENT. As a result, upon comparing equations (1) and (2), it is clear that an unavoidable degradation of beam quality will result both from the difference between the focussed spot size S.sub.F and the fiber diameter d.sub.f as well as from the cone angle difference.
The adverse effect of fiber transmission on beam quality has a direct effect on the power density achievable at the spot focussed on the workpiece. This is because the size of the spot that can be focussed on the workpiece is dependent on the beam quality of the beam being focussed. Referring again to FIG. 1 which illustrates prior art output coupler 100, lens 108 collimates the fiber emitted beam to a diameter D.sub.1. Recalling that workpiece 118 is positioned such that spot 116 is spaced from focussing lens 112 by the distance f.sub.2 , then the diameter of spot 116, represneted by d.sub.s , is given by: ##EQU1## where BQ.sub.F is the beam quality of the laser beam as emitted from the optical fiber. Thus, the focussed spot size depends directly on the beam quality so that to the extent the beam quality is degraded by fiber transmission, the value of BQ.sub.F increases, the focussed spot size increases and the achievable focussed spot power density decreases. It is noted that this adverse effect of beam quality degradation is amplified by the variation of focussed spot power density as the inverse of d.sup.2.sub.5.
The limitations inherent in the configuration of the prior art output coupler, for maximizing focussed spot power density, are described next. As seen in equation (3) above, the ability to adjust the design of the prior art output coupler to minimize the focussed spot diameter, and thereby maximize focussed spot power density, is limited to decreasing the focussing lens focal length and/or increasing the beam diameter (D.sub.1). With respect to reducing the focussing lens focal length, there are practical processing considerations regarding a need to maintain a safe distance between the output coupler and the workpiece. If the coupler is too close, products given off by the process, e.g. weld spatter, may impinge on the coupler and cause damage thereto. Also, as known in the art, there are theoretical and practical manufacturing limitations on lenses which, for a given lens clear aperture, set the minimum possible focal length. For example, the approximate minimum ratio of focal length to clear aperture is "1" for an aspheric lens and "2" for a plano-convex lens. With respect to effecting an increase in beam diameter by adjusting the prior art coupler, two restrictions apply. First, such an increase in beam diameter would be accomplished by selecting collimating lens 108 with a longer focal length of intercept the fiber emitted beam at a larger diameter. However, interception of the larger diameter beam will likely also require a larger clear aperture for the collimating lens. Depending on the magnitude of .theta..sub.EM, the required increase in lens clear aperture may outpace the required increase in lens focal length such that the above noted limitations on lens manufacture render fabrication of a suitable collimating lens impossible. Second, to the extent the clear aperture of the collimating lens is less than the diameter of the expanding beam, a portion of the total beam power will be lost and a corresponding reduction in achievable focussed spot power density experienced.
The above described possible adjustments to the prior art output coupler lenses can, in any event, achieve a focussed spot diameter that is no less than the diameter of the fiber from which the beam is emitted. Referring to FIG. 1, the emitted cone angle .theta..sub.EM can be expressed as: ##EQU2## and this result can be substituted into equation (2) to provide: ##EQU3## for the beam quality of the fiber emitted beam. The beam quality BQ.sub.S1 of the beam focussed by lens 112 can be derived in analogous fashion and expressed as: ##EQU4## Since the beam qualities BQ.sub.F and BQ.sub.S1 must be identical, the right hands sides of equations (5) and (6) can be set equal to one another and then solved for spot diameter d.sub.s to provide: ##EQU5## It is therefore desirable to minimize the ratio f.sub.2 /f.sub.1 in order to minimize spot diameter d.sub.s. However, the instant inventor observes that since lenses 108 and 112 are respectively characterized by the lens manufacturing ratios f.sub.1 /D.sub.1 and f.sub.2 /D.sub.1, the ratio f.sub.2 /f.sub.1 cannot be reduced below "1". As a result, the focussed spot diameter d.sub.s can in no event be reduced below d.sub.f.
It is therefore a principal object of the present invention to provide an output coupler, for use with a power laser beam transmitting optical fiber, that compensates for the above described adverse effects of fiber transmission in order to improve the focussed spot power density achievable with the fiber transmitted beam.